Cooling of different elements in plasma etching systems is important in order to achieve consistent etching over a large number of operations and to optimize etch results. For example, it is common to provide liquid cooling to the counter electrode and to the chuck electrode.
A wafer including a film to be etched and generally disposed on the chuck electrode is subjected to a heat flux resulting from the plasma within the reactor. The heat flux comprises a large fraction of the R.F. power delivered to the reactor. In the course of etching, the wafer temperature rises until the heat transfer to the chuck equals the heat input from the plasma.
The temperature of the wafer surface is a significant variable in many plasma etch processes. Excessive and variable temperatures have a great effect on resist degradation and process selectivities. Reliable control of wafer temperature rise and uniformity is difficult to achieve because of variability in the heat input to the wafer and inadequacy of the cooling processes which limit the temperature rise. Also, wafer temperature has not been independently controllable in the past. Instead, the temperature of the wafer has depended to an important degree upon such parameters as R.F. power, process gas pressure, and wafer flatness and has frequently been excessively high.
A well-recognized source of the problem of cooling the wafer is the difficulty in efficiently transferring heat from the wafer to the chuck electrode when low gas pressures and low contact pressures are involved. The wafer temperature can easily rise 100.degree. C. or more above that of the chuck electrode which makes very difficult to achieve effective control by way of the chuck electrode temperature even when the chuck electrode is independently cooled. Also, the time constant of the thermal equilibration is tens of seconds when the thermal clamping is weak, which means that the wafer temperature is not even approximately constant during the etch.